The present relates to a novel chromium-free etching solution suitable for the treatment of silicon substrates as well as silicon germanium substrates, as well as methods for treating such substrates and methods for revealing defects on such substrates using the etching solution in accordance with the present invention.
Crystalline defects in substrates for microelectronic devices are highly undesirable as they have a negative impact on the functionality and reliability of integrated circuits, formed using the substrates, such as wafers, in particular silicon-on-insulator (SOI) type wafers. The typical approach for identifying crystalline defects and thereby characterizing the quality of substrate surfaces is the use of so-called structural etching solutions. These etching solutions, due to the dependency of the etch rate from crystalline structures, can identify crystalline defects, since crystalline defects give rise to either hillocks or etch pits after application of the structural etching solution.
Various etching solutions have been proposed for silicon surfaces which require typically the presence of strong oxidants. F. Secco describes in Journal of Electrochemical Society, 119, no. 7, pp. 948-951 (1972) an etching solution for revealing etch pits in silicon, that includes a mixture of hydrofluoric acid and aqueous alkali dichromate. The alkali chromate acts as oxidizing agent while the hydrofluoric acid dissolves the oxidation product, namely silicon dioxide. However, chromates and, in particular, dichromates are highly toxic due to their ability to interact with cells and DNA.
W. C. Dash in the Journal of Applied Physics, vol. 27, no. 10, pp. 1193-1195 (1956) discloses a further etched solution able to reveal defects on semiconductor substrates, consisting of hydrofluoric acid, nitric acid and acetic acid. While this solution is able to etch semiconductor substrates, including silicon substrates, the etching solution according to Dash is not able to differentiate between different types of defects and furthermore does not provide a satisfactory etch rate.
U.S. Pat. No. 2,619,414 discloses a further chemical etchant to be applied on semiconductor surfaces to improve their electrical characteristics. The chemical etchant disclosed in this patent comprises acetic acid, nitric acid, hydrofluoric acid and bromine. The drawback of this composition is the use of bromine, which is highly instable and volatile, so that the chemical etchant according to this prior art reference can only be stored for a very short time in the dark at low temperatures and can be handled only under ventilation, since bromine evaporates from the composition. Although bromine is not as toxic as chromate or dichromate, precautionary measures nevertheless have to be taken when using the chemical etchant according this patent.
With respect to substrates comprising in addition to silicon also germanium, similar etching solutions have been proposed in the prior art. D. G. Schimmel discloses in J. Electrochem. Soc., Solid State Science and Technology, 479-483 (1979), the use of a chromium comprising solution, mainly characterized by a ratio of HF/CrO3 of 1:1. The major drawback of this etching solution is again the presence of highly toxic chromium. Other chromium containing etching solutions are the solutions according to Wright, comprising HF, HNO3, CrO3, Cu(NO3)2, water and acetic acid. Again, the major drawback of such an etching solution is the use of highly toxic heavy metals, in particular chromium (M. W. Jenkins, J. Electrochem. Soc., vol. 124 (1977), 757-762).
In view of the above-identified drawbacks of the conventional etching solutions, attempts have been made in order to propose etching solutions being chromium-free. As already indicated above, the etching solution according to Dash comprises hydrofluoric acid, nitric acid and acetic acid, namely HF (concentration 49%), HNO3 (concentration 70%) and acetic acid (concentration 100%) in a volume ratio of 1:3:10. However, as already indicated above, the etch rate obtainable with the etching solution in accordance with Dash is not satisfactory. Other chromium-free etching solutions are, for example, the etching solutions according to Sopori consisting of hydrofluoric acid (concentration of 49%), nitric acid (concentration 70%), acetic acid (concentration 100%) at a volume ratio of 36:1-2:20. Again, the etch rates obtainable with this etching solution are not satisfactory so that this etching solution in fact only has been employed for treating polysilicon (B. L. Sopori, J. Electrochem. Soc. vol. 131 (1984), 667).
Finally, a chromium-free etching solution has been proposed as solution B in the Japanese standard JISHO 609-1999. This solution consists of hydrofluoric acid, nitric acid, acetic acid and water at a volume ratio of 1:12-7:3:5.7. Again, it has to be noted that this etching composition does not provide a suitable etch rate and it furthermore has been determined that this etching solution does not give rise to a homogenous etching so that satisfactory etch result cannot be obtained.
In view of the drawbacks as identified above and the progress semiconductor industry, involving in particular the decrease of the minimum feature sizes used to fabricated integrated circuits, the introduction of new substrate materials, such as silicon on insulator (SOI) or strained silicon on insulator (sSOI), improved etching solutions are required in order to enable suitable quality characterization, in particular with respect to the following features:
satisfactory etch rates, so that even thin substrates can be etched with a sufficient control of etch rate/etch time, removed surface thickness.
etch homogeneity, i.e. homogenous etching without stain generation etc.
etch sensitivity, i.e. the possibility to detect different types of defects (including D defects corresponding to agglomerates of vacancies and oxygen precipitates, as well as bulk stacking fault (BSF) and bulk micro defects (BMD)).
reduction of health risks and environmental problems by using suitable components for the etching solution, without sacrificing the desired properties, such as etch rate, etch sensitivity etc.
stability of the etching solution, so that same can be stored for a certain period of time and can be handled without highly elaborated safety measures.
suitability for different substrates, including silicon substrates as well as silicon germanium substrates.
This need is now addressed by the present invention.